This invention relates generally to precision drilling operations, and more specifically, to a collet clamping nosepiece for power feed drilling equipment.
Many applications, such as automotive and aircraft fabrication, require precisely positioned holes to be drilled into a workpiece. The holes must be accurately aligned with only relatively small positioning errors tolerable. Accordingly, a drill plate is typically aligned and mounted in a fixed relation to the workpiece prior to drilling holes in the workpiece. The drill plate has one or more accurately positioned holes extending therethrough such that by properly aligning the drill plate with respect to the workpiece, a hole in the drill plate precisely overlies the desired location for a corresponding hole to be drilled in the workpiece.
Traditionally, the assembly of aerospace products involves mounting power feed drill motors to dedicated drill plates which establish hole location and normality with respect to the product being assembled, react to drilling thrust, and support the weight of the drill motor. Therefore, the operator of the drill does not have to offset the full weight of the drill motor and the thrust generated during the drilling operation. Attachment of the drill motor to the drill plate further ensures proper alignment of the cutting tool of the drill with the desired hole location in the workpiece. In many instances, the accurately positioned holes must be drilled in relatively hard materials, such as titanium. To drill holes in such materials, it is desirable to use power feed or rack feed drill motors which produce relatively great amounts of axial thrust. Typically, these motors are configured such that the drill spindle and cutting tool are actuated with pneumatic and/or hydraulic power, or with the assistance of a rack and pinion drive. Since these motors are generally heavy and produce a relatively great amount of thrust, however, the drill motor is generally firmly clamped to a drill plate and the drill plate is, in turn, firmly clamped to the workpiece. Accordingly, the affixation of a drill motor to a drill plate eliminates the need for the drill motor operator to apply axial pressure during the drilling operation.
To enable the drill motor to be easily mounted to the drill plate in any orientation, colleting mechanisms have been used as integrated components of the drill motor or added as a separate colleting nosepiece accessory. These collet clamping mechanisms are frequently bulky, require special length cutters and in many instances are awkward to remove, service, and use.
Each integrated collet clamping mechanism only works for one specific drill motor. Consequently, colleting nosepieces that could be added as an accessory to several drill motors were developed. However, these nosepieces must produce clamping forces in the range of 270 lbs to 850 lbs to securely mount the drill motor to the drill plate. To provide such clamping forces, bulky pneumatic pistons have been incorporated into nosepiece designs that either make the above mentioned cutter very long, or create a large obstacle at the end of the drill motor that prevents access to numerous hole locations on aircraft structures.
One previous attempt to produce a collating nosepiece is documented in U.S. Pat. No. 5,395,187. The device described in this patent is rather large and bulky. In the exemplary embodiment, the overall length was 7.25 inches, the diameter was 5.75 inches, and requires a ten inch long cutter. The tip to the nosepiece is relatively slender at 1.25 inches in diameter, but is only 2.5 inches long. The relatively short length is not long enough to clear standard length wedgelocks. Consequently, this device has seen only limited implementation.
Another collating nosepiece is documented in U.S. Pat. No. 5,482,411 has been an improvement over the above described collating nosepiece. The outside diameter of the collating nosepiece was reduced to three inches, due to the use of two air pistons on the same piston rod, and the overall length was reduced to about 6.5 inches. It has been implemented in at least one aircraft structure application, but still has disadvantages. For example, this colleting nosepiece has an outside diameter of about three inches which limits the areas of aircraft structure that it can access and frequently interferes which wedgelocks, which are typically used to clamp aircraft structure together prior to drilling. This colleting nosepiece also requires the utilization of relatively expensive nine inch long twist drills due to the location of the air pistons that provide the clamping force.
Additionally, the drilling debris must travel approximately 3.5 inches to a vacuum collection chamber causing clogs and excessive down time. In order to change the collet or the mandrel of the colleting nosepiece described by U.S. Pat. No. 5,482,411, the entire nosepiece must be removed and disassembled. Removal and disassembly is a time-consuming, and therefore expensive, process.
This collating nosepiece also requires utilization of drill plates with a specific thickness and a specific drill plate liner, limiting the implementation of the collecting nosepiece. This collating nosepiece also relies on the surface of the drill plate for normality. It is fairly difficult to provide the required normal surface on highly contoured drill plates and adds a spot-facing operation to the cost of the drill plates. Finally, the drill bushing must be spring-loaded against the mold line surface to form a vacuum seal. For this to occur, the operator must push on the back of the drill motor set-up to compress the spring, holding it in place while the operator activates the colleting nose. This situation is an ergonomic issue and frequently causes improper clamping of the drill motor to the drill plate resulting in oversized holes.
Another known concentric collet nosepiece which has an overall length of about 3.5 inches, but this length is accomplished by situating the clamp piston off to the side of the expansion mandrel and the collet. The clamping force is transmitted to the collet through two pivoting clamp arms. The clamp piston and the associated mechanism extend about 4.15 inches from the centerline of the twist drill and are an obstacle that interferes with adjacent tooling, structures, and wedgelocks.